Nonlinear interactions of internal waves and the force of internal solitary waves on small - Diameter cylinders

This study utilized high-quality mooring data collected in August 2015 near the Dongsha Islands in the northern South China Sea to investigate the nonlinear interactions of internal tides and the evolution of internal solitary waves. By applying both the Morison equation and its modified equation, the forces exerted by internal solitary waves on small-diameter cylinders under the influence of internal tides were also computed, and the applicability of the two approaches was assessed. In addition to identifying the forced non-resonant interactions, this study also revealed the presence of the semi-diurnal tidal parametric subharmonic instability (PSI) mechanism. For example, a PSI resonant triad can be formed by two M1 waves and one M2 waves. Besides, the diurnal internal tides were predominantly governed by the third mode, while the semi-diurnal internal tides were primarily dominated by the first mode. During the observation period, internal solitary waves recurred every 25 h, with amplitudes up to 63 m and horizontal velocities exceeding 1 m/s. The forces acting on the cylinder were found to be primarily influenced by the first mode, peaking at the wave trough. The modified Morison equation better captured the nonlinear effects of internal solitary waves compared to the original Morison equation.